The present invention relates to a coating method and to the product resulting from such coating method. Such product is an ideal candidate for the application of a cathodic electrophoretic coating or e-coat.
Briefly, the coating technology to which the present invention relates is described in the literature under such terms as cathodic electrodeposition, cathodic electrophoretic coating, or e-coating. Such technology was developed in the mid-1970's and is now widely practiced in the automotive and appliance industries. While there may be other and varied applications for this technology, for convenience herein, reference will be made to the problems and advances made in the automotive industry as this is an industry which has experienced the greatest pressures to perfect such technology.
The automotive industry is under increasing pressure to meet the demands of the consuming public wanting automobiles that "won't rust", yet presenting a pleasing appearance. Supporting the public is the government with its potential power to demand higher quality in the form of a longer term warranty. Two parties helping to relieve such pressure are the steel industry which supply the steel sheet for the automobile bodies, and the paint industry which supply the eye-appealing and corrosion-resistant paint for such bodies.
The automotive industry has adopted cathodic electrodeposition as a coating method for a number of reasons. Such reasons include the ability to obtain uniform coverage of the substrate, access to all parts of the substrate, increased corrosion protection afforded by cathodic primers, and automation, by way of example. One of the disadvantages or conditions of coating through cathodic electrodeposition is that the substrate must be electrically conductive such as found with steel. Although cathode electrocoat primers provide a degree of corrosion protection, paint on bare steel will not be sufficiently corrosion resistant to satisfy either the consumer or the government in achieving the long awaited "rust-free" automobile. Accordingly, the steel industry and the paint industry have tried a number of approaches to provide a steel product which has a corrosion resistant coating that will be receptive to the application of an e-coat.
One approach to the problem proposed by the steel industry was the use of two sided galvanized steel as a base product to minimize inside-out corrosive attack of the auto body. These attempts failed because the painted part could not meet the automotive industry's criteria for appearance. The steel industry then turned to a zinc-rich paint system applied to only one side of a steel strip on a continuous coil coating paint line. A commercial product utilizing such a system is ZINCROMETAL, a registered trademark of Diamond Shamrock Corporation. ZINCROMETAL is actually a dual cost system wherein the initial coat is a proprietary mixture of chromic acid, zinc dust and other chemicals, while the outer coating is an organic resin containing zinc powder. While ZINCROMETAL coatings appeared to satisfy the requirement for providing adequate protection against the corrosive effects of road salt, such coatings tended to show an inherent surface defect when cathodic electroprimed at high voltages. By high voltages we mean voltages in excess of 250-300 volts, as typically used in the U.S. automotive industry. In any case these surface defects had the appearance of craters or pinholes in the surface. The subsequent applied outer coating was not sufficient to mask the craters. As a consequence, such coatings were restricted to the non-visible area of an automobile.
The cratering problem is a topic of world-wide interest as evidenced by the following articles.
1. "Problems Associated with the Electrophoretic Deposition of Paint on Galvanized Steel," by L. W. Franks et al, presented at ASM/ADDRG Conference in April 1981 at Dearborn, Mich., and PA0 2. "Multilayer ElectroGalvanized (Zn--Cr--CrOX) Steel Sheet for Optimum Corrosion Protection of Car Bodies," by a. Catanzano et al, presented at SAE Int'l Conference in February-March, 1983 at Detroit, Mich.
In the Franks et al article, cratering is attributed to hydrogen generation. The authors identify two factors with cratering, namely, deposition voltage and deposition current density. Catanzano et al offer an extensive discussion on `Hydrogen Cratering`. However, rather than attempt to modify the operating conditions of the process, the latter authors propose a multilayer electrogalvanizing process. The result of such process is a coated product, allegedly resistant to cratering, which was given the name ZINCROX, a registered trademark of Zincroksid S.p.A.
Notwithstanding the above work, the present invention is based on the discovery that cratering is not related to hydrogen evolution. By understanding the causes of cratering, it was possible to develop a method for providing a corrosion resistant coating which is not susceptible to cratering when coated with a cathodic electrophoretic primer at voltages in excess of 300 V. Such development, to be described in detail in the specifications which follow, can open the door to the use of e-coats to the visible areas of an automobile.